1. Field of the Invention
This invention relates generally to improved techniques for producing alpha-olefins from low sulfur hydrocarbon fractions and more particularly to the production of ethylene from low sulfur naphthas. More specifically, the invention relates to a process for converting a source of natural gas at a remote site into a low sulfur naphtha, transporting the low sulfur naphtha to a different facility and subsequently processing the low sulfur naphtha to produce ethylene.
2. Description of the Related Art
The conversion of hydrocarbons into olefins such as ethylene and propylene is an important industrial process which produces billions of pounds of olefins each year. Important sources of hydrocarbons for conversion include natural gas and refinery gases. In one industrial process, methane, which is a major component of natural gas, is steam reformed and/or partially oxidized to produce synthesis gas which is primarily composed of CO and H2. Synthesis gas can be processed via known Fischer-Tropsch (FT) syntheses which involve converting the gas in the presence of a catalyst into hydrocarbon products suitable for conversion into olefins such as ethylene.
Natural gas is often plentiful in remote areas that are uneconomical to develop because of the lack of local markets for the gas or the high cost of transporting the gas to areas of high demand. One approach would be to convert the gas to a liquid chemical product for more cost-effective transportation to existing industrial facilities for further processing.
Ethylene is a valuable intermediate widely used in the production of such diverse materials as plastics, ethylbenzene, styrene, ethylene glycol, ethylene oxide, etc. Most of the ethylene produced world-wide is made by thermal cracking of light ethylene cracker feedstocks composed of hydrocarbons ranging from ethane to C5-C10 naphthas. If ethane is the feed, the unit is typically referred to as an ethylene cracker. If the feed is a mixture of ethane and propane, the unit is typically referred to as an EP or ethylene-propylene cracker since the unit produces ethylene and propylene. When the feedstock is a naphtha, the unit is called a naphtha cracker because of the ability to process heavier feeds. Water is conventionally added to the feeds in these units to lower partial pressure and minimize polymerization.
The yield of ethylene from a naphtha cracker depends upon the hydrocarbon content in the feed. Ethylene yields are highest when the feed is composed of high concentrations of paraffins, particularly linear paraffins. However, only limited supplies of highly linear paraffin feedstocks are available from petroleum refineries. When conventional naphthas (i.e. unrefined naphthas) are processed in the cracker units, the downstream facilities that separate the ethylene and propylene streams must be appropriately sized to compensate for the relative production of ethylene vs. heavier products. When highly linear paraffin hydrocarbon feeds such as Fischer-Tropsch naphthas are processed in naphtha cracker units, the ethylene yields may be so high compared to other products that the capacity of the facility to process the ethylene stream volume is sorely taxed. Also, the production of valuable by-products may be compromised.
It has also been observed that olefin cracker units suffer from furnace tube fouling caused by reaction between carbon and the metal of the furnace tube. The result is a decline in the heat transfer coefficient and the formation of soot and metal dust. Eventually the integrity of the furnace tube becomes so compromised that the tube needs to be replaced before rupturing. One means of minimizing this problem has been to add organic sulfur compounds to the feedstock. Because sulfur, even in small amounts, is poisonous to conventional Fischer-Tropsch catalysts, processing techniques have been employed to remove sulfur, including sulfur-containing compounds, upstream of the Fischer-Tropsch synthesis. As a consequence, Fischer-Tropsch distillates normally are essentially free of sulfur or sulfur-containing compounds. Processing these products in naphtha crackers could lead to furnace tube fouling.
One proposal for converting natural gas at remote sites to ethylene has involved converting natural gas into synthesis gas and then into methanol for transport to existing facilities for conversion into ethylene. The problem with this approach is that the production of methanol is expensive and the transportation of methanol would create an environmental hazard if spillage occurs. Furthermore, the proposal would necessitate the construction of new facilities to convert methanol to ethylene and propylene and existing naphtha crackers could not be used.
Information on the cracking of hydrocarbons to produce ethylene may be found throughout the patent and technical literature. Reference may be made to the following: xe2x80x9cEncyclopedia of Chemical Technologyxe2x80x9d, 4th edition, Kirk-Othmer, volume 9, pages 883 to 900; William A. Gruse and Donald R. Stevens, xe2x80x9cChemical Technology of Petroleumxe2x80x9d, 3rd Edition, McGraw-Hill, 1960, pages 344-348.
U.S. Pat. No. 6,075,061 discloses a process wherein natural gas is separated into first and second sulfur-containing hydrocarbon streams, the second stream de-sulfurized, subjected to a syngas generation, the syngas subjected to a hydrocarbon synthesis, and the hydrocarbons then subjected to hydrotreating and hydroisomerization in the presence of the first stream. U.S. Pat. No. 6,180,842 discloses stabilizing a Fischer-Tropsch derived distillate having a sulfur content of less than 1 ppm by adding a virgin distillate having a sulfur content of at least 2 ppm. Neither of these patents is concerned with ethylene production or furnace tube fouling.
Accordingly, a need exists for a process for producing ethylene and propylene by utilizing natural gas sources at remote sites for conversion of methane into low-sulfur liquid hydrocarbon products, and transporting the liquids to existing industrial facilities to be used as feedstocks in naphtha-crackers while not suffering a reduction in the production of propylene and heavier olefinic by-products and avoiding cracking problems caused by furnace tube fouling.
It is therefore an object of the present invention to develop an efficient process for producing ethylene at an industrial site utilizing methane obtained from a remote natural gas site.
It is another object of the invention to prepare alpha-olefins in naphtha cracker units using low sulfur paraffinic hydrocarbon feeds while avoiding furnace tube fouling.
It is still another object of the invention to produce feedstocks for naphtha cracker units comprising blends of low sulfur Fischer-Tropsch naphthas with sulfur-containing compounds or sulfur-containing petroleum naphthas which avoid the aforementioned problems.
These and other objects of the present invention will become apparent to the skilled artisan upon a review of the following description and the claims appended thereto.
These and other objects and objectives of the invention are attained by providing a process for producing ethylene comprising converting methane at a natural gas-producing remote site into synthesis gas, converting the synthesis gas via a Fischer-Tropsch synthesis into low sulfur paraffinic hydrocarbon liquids containing less than 1 ppm sulfur, transporting the liquids to a developed industrial site, adding a sulfur-containing compound or a sulfur-containing naphtha to the liquid to form a feedstock blend containing at least 1 ppm sulfur, converting the blend in a naphtha cracker unit to ethylene, and recovering the ethylene and other by-products therefrom. For it has been discovered that by converting methane at a remote site to a synthesis gas, and then converting the synthesis gas via Fischer-Tropsch procedures into a low-sulfur paraffinic hydrocarbon liquid containing less than 1 ppm sulfur, the low-sulfur naphtha can be effectively transported to a developed facility. Further, by then adding a sulfur-containing compound or a mixture containing a sulfur compound to the low-sulfur naphtha to provide a blend containing more than 1 ppm sulfur, coking problems are avoided. Thus, the addition of the sulfur-containing compound or mixture allows one to avoid coking problems and the reduction in the production of propylene and heavier by-products that would normally be observed with a Fischer-Tropsch naphtha alone. The result is an efficient and effective process for converting methane from a remote site into ethylene.